The present invention relates to a biaxially stretched polyester film exhibiting a low degree of curling, and particularly to a biaxially stretched polyester film which exhibits a very low degree of curling when one side thereof is dipped in a solvent.
Since biaxially stretched polyester films exhibit excellent heat resistance, mechanical properties, chemical resistance and so on, they are used in a wide range of applications and the demand for such films increases year by year.
Although such films have excellent characteristics, if one side thereof is dipped in a solvent and then dried, curls occur. This curling in some cases leads to restrictions in use, depending upon the use concerned. For example, in a process of removing a polyester layer from a photosensitive layer of a solvent development-type photo resist film by treatment with methylene chloride before development, the film curls toward the photosensitive layer side. A high degree of curling interferes with development in some cases and is thus a very critical problem.
When polyester films are used as the base films of magnetic recording media, an undercoating is provided, if required, on the polyester films by using p-chlorophenol or the like as a solvent. In this case, there is a problem that one side of the polyester film becomes swollen and thus becomes curled.
As described above, polyester films are treated with a solvent more and more frequently in various applications thereof, and there is thus a great demand for development of a polyester film which is capable of coping with this.
As a result of the research conducted by the present inventors in consideration of the above-described problem, the present inventors have found that a polyester film having particular physical properties has resistance to curling. The present invention has accomplished based on this finding.